1. Field of the Invention
The present invention relates to methods of determining a quality of a light source, and more particularly related to methods of determining a quality of a light source applied to a photolithographic process.
2. Description of the Related Art
With advances of electronic products, semiconductor technology has been widely applied in manufacturing memories, central processing units (CPUs), display devices, light emitting diodes (LEDs), laser diodes and other devices or chip sets. In order to achieve high-integration and high-speed specifications, dimensions of semiconductor integrated circuits have been reduced and various materials and techniques have been proposed to achieve these targets and overcome obstacles during manufacturing. To scale down dimensions of integrated devices and circuits, photolithographic technology, e.g., exposing process, plays an important role.
The exposing process involves a light source which dominates the resolution of feature dimensions of integrated circuits. The desired resolution can be obtained by increasing the depth of focus (DOF) of the light source, the quality of the light source or other conditions of the exposing step. Accordingly, methods to determine the quality of the light source have been proposed.
FIGS. 1A and 1B are pictures of pupil maps. A pupil map is an image of light from a light source impinging on an image sensor array, used to determine whether the light source is acceptable to be applied to a photolithographic process. To generate the pupil maps, a light is provided from a light source module. The light passes through a shield and is projected on a charge-coupled device (CCD) array. The CCD array comprises a plurality of CCDs. Each of the CCDs senses a respective intensity (gray level value) and represents an address in the CCD array. Thus, the CCD array creates the image of the pupil map corresponding thereto.
Traditionally, an engineer or operator determines the quality of the light source based on visual inspection and personal experience. A skilled engineer or operator can distinguish the pupil map of FIG. 1A from the pupil map of FIG. 1B due to distortion, voids or discontinuity of the pupil map in FIG. 1B. The pupil map of FIG. 1A is acceptable, but the pupil map of FIG. 1B is not acceptable. The conditions of the light source module which generate the pupil map of FIG. 1B are modified to create a desirable pupil map. If distortions, voids and discontinuity of the pupil map are substantial, but not so serious that it is difficult to determine the quality of the light source, some undesirable pupil maps may be incorrectly determined to have acceptable quality. The conditions of the exposing step of such pupil maps are then used for performing a photolithographic process. Such conditions of the exposing step will generate undesired photolithographic patterns and affect subsequent processes.
From the foregoing, improved methods of determining the quality of a light source are desired.